The present invention relates to a nitride system semiconductor device usable for a semiconductor laser and a light emitting diode and a method for manufacturing the same and, more particularly, to a low-resistance nitride system semiconductor device having good ohmic contact with an electrode and operated at a low voltage and a method for manufacturing the same.
A nitride system semiconductor such as GaAlN, from GaN down, has recently been noticed as materials for a light emitting diode (hereinafter referred to as LED) and a semiconductor laser diode (hereinafter referred to as LD) in a short wavelength region from blue to ultraviolet. In particular, the InGaAlN system mixed crystal has the maximum transition type energy gap in the III-V family compound semiconductor mixed crystal, and is known as light emitting materials in a wavelength of 0.2 .mu.m to 0.6 .mu.m or in a region from red to violet.
A semiconductor light emitting device using the above nitride system semiconductor materials necessitates a p-type conductive layer and an n-type conductive layer, constituted of a nitride type semiconductor layer as a current injection layer and a contact layer contacting an electrode.
In manufacturing a current injection type light emitting device, it is essential to control the conductivity type, conductivity (impurity concentration, carrier concentration), etc. of each of the p- and n-type conductive layers since the element is based on a pn junction type. In InGaAlN system materials, the conductivity type of the n-type conductive layer can be relatively easily controlled by using silicon (Si) as impurities.
On the other hand, it is difficult to control the conductivity type, conductivity (accepter concentration, carrier concentration), etc. of the p-type conductive layer. As one method of forming the p-type conductive layer, magnesium (Mg) or zinc (Zn) is usually employed as dopant, a growth substrate held at a high temperature of about 1100.degree. C. is placed in hydrogen carrier gas (H.sub.2) and ammonia gas (NH.sub.3), and Ga and Al raw materials are supplied onto the growth substrate.
However, an Mg-doped nitride system semiconductor layer such as an Mg-doped GaN layer and an GaAlN layer, which is formed by the above method, exhibits high resistance, not p-type conductivity.
It is thought that acceptors of Mg and Zn are prevented from being activated by deep level of impurities of Zn and Mg and by active hydrogen atoms, which are dissolved from ammonia (NH.sub.3) as material gas and hydrogen as carrier gas, or the other residual impurities particularly in the MOCVD (Metalorganic Chemical Vapor Deposition)(J. A. Van Vechten et al., Jpn. J. Appl. Phys. 31, 1992, 3662).
If the Mg-doped InGaAlN layer is grown by the MOCVD, hydrogen is taken into crystal from ammonia (NH3) and carrier gas, together with magnesium (Mg), before the temperature of the substrate is decreased to the room temperature during or after the growth of an Mg-doped layer, and Mg acceptors will be inactivated by H+, thus increasing the resistance of the Mg-doped InGaAlN layer. If, for example, a GaN layer is doped with Mg at a concentration of 1.times.10.sup.20 cm.sup.-3, the concentration of hydrogen is also taken into the GaN layer by 1.times.10.sup.20 cm.sup.-3 which is same degree as that of Mg. The hydrogen concentration of the Mg-doped GaN layer is ten or more times as high as that an undoped or Si-doped GaN layer grown under the same conditions, and it was confirmed by Hall measurement, C-V measurement or the like that the grown Mg-doped GaN layer was increased in resistance.
The Mg-doped GaN layer, which is grown and increased in resistance, is subjected to electron irradiation (H. Amano et al., Jpn. J. Appl. Phys. 28, 1989, L2112) or heat treatment (S. Nakamura et al., Jpn. J. Appl. Phys. 31, 1992, 1258) to thereby to improve the activation of Mg and achieve a practical high-luminosity device and LD emission (S. Nakamura et al., Jpn. J. Appl. Phys. 35, 1996, L74).
In general, the Mg-doped nitride system semiconductor layer undergoes post-treatment such as heat treatment for removing hydrogen in an atmosphere of gaseous nitrogen of 600.degree. C. to 800.degree. C.
After the heat treatment, the Mg-doped nitride system semiconductor layer exhibits a p-type conductivity type but causes a high-resistance layer on the uppermost surface of a growing layer. The reason why this high-resistance layer is caused, will now be described in detail.
FIG. 1 is a schematic view showing the structure of a typical blue semiconductor laser using a nitride system semiconductor. The blue semiconductor laser has a multilayer structure in which a buffer layer (not shown), a GaN underlying layer 2, a GaN contact layer 3, an n-type GaAlN current injection layer 4, an active layer 5 having a multiple quantum well (MQW) structure using InGaN, a p-type GaAlN current injection layer 6, and a p-type GaN contact layer 7 for forming a p-type electrode are formed in sequence on a sapphire substrate 1 by the MOCVD.
When the multilayer structure blue semiconductor laser is formed, hydrogen is used as carrier gas for all the layers except the InGaN system active layer 5. On the other hand, nitrogen is employed as carrier gas in forming the InGaN active layer 5. When the p-type GaAlN current injection layer 6 and p-type GaN contact layer 7 are formed, Mg is used as p-type dopant. Since Mg is not activated during the growth, the multilayer structure is subjected to heat treatment in the nitrogen atmosphere.
After the heat treatment, part of the multilayer structure is removed to such a depth as to reach the GaN contact layer 3 by dry etching and then an n-side electrode 8 is formed on the GaN contact layer 3. A p-side electrode 9 is formed on the unremoved part of the p-type GaN contact layer 7. Thus, the sample including these electrodes 8 and 9 is cleaved to form facets for the laser cavities, resulting in a blue semiconductor laser.
Since, however, the blue semiconductor laser has a high-resistance portion on the uppermost surface of the contact layer 7, its operation voltage is high, thus making it difficult to inject current necessary for laser emission into the device. If the current is to be injected, then the operation voltage will increase to 20V or higher and the vicinity of the p-side electrode 9 will be broken. To resolve this problem, the contact resistance of the p-side electrode 9 has to be lowered.
In the blue semiconductor laser, the distribution of concentrations of magnesium (Mg), carbon (C), hydrogen (H) and oxygen (0) in the depth direction of the sample before and after the heat treatment in the nitrogen atmosphere, was obtained by secondary ion mass spectrometry (hereinafter referred to as SIMS). As a result, as shown in FIG. 2, the concentration of Mg is fixed in the depth direction of the sample both before and after the heat treatment. On the other hand, the distribution of concentrations of carbon (C), hydrogen (H) and oxygen (O) is virtually constant before the heat treatment, whereas, after the heat treatment, a larger amount of carbon (C), hydrogen (H) and oxygen (O) is detected on the uppermost surface of the growing layer of the sample than that inside the growing layer. For example, the carbon and hydrogen are each sometimes detected on the uppermost surface of the growing layer more than that inside the growing layer by one to two figures.
The causes of increasing in resistance of contact between the p-type GaN contact layer 7 and p-side electrode 9 and thus heightening the device voltage, are as follows. Hydrogen diffuses from inside the growing layer onto the surface thereof by heat treatment, and a large amount of hydrogen remains on the uppermost surface thereof, with the result that the hydrogen is combined with magnesium and the magnesium is inactivated. There is a large amount of carbon on the uppermost surface of the growing layer due to contaminant caused when the carbon is diffused from inside the growing layer or the heat treatment is carried out, while there is a large amount of oxygen due to a surface oxide film caused by heat treatment or its diffusion from inside the growing layer.
If there is a large amount of oxygen or carbon on the uppermost surface of the growing layer, the following three problems (1) to (3) will arise.
(1) The activation rate of dopant such as Mg is decreased, as is the density of carriers, and (2) no good ohmic contact with the electrode can be achieved. These problems (1) and (2) decrease the reliability of the device, i.e., they increase the resistance of contact with the p-side electrode 9, increase the operation voltage of the device, shortening the lifetime of the device, etc. Furthermore, (3) the impurities are diffused by crystal defects such as a dislocation and a stacking fault, and the leak current will be increased.
The nitride system semiconductor device described above necessitates heat treatment; however, the time required for sufficiently activating magnesium in the growing layer is varied with a difference in device structure such as the thickness of the p-type layer and the composition of each layer. Though the heat treatment time can be estimated to some extent taking into consideration the diffusion of hydrogen, it is not easy to determine the optimum heat treatment time at a heat treatment temperature.
In other words, though the nitride system semiconductor device requires the heat treatment, the heat treatment temperature and time the most suitable for dissociating hydrogen are indefinite, the hydrogen remains in the p-type conductive layers 6 and 7, or an excessive heat treatment, and there are strong possibility that the activation rate of magnesium is decreased. The hydrogen remaining in the layers 6 and 7 causes the resistance of the entire device to increase and, like the contact resistance, it causes the operation voltage of the device to rise.
In order to solve the above problem, a trial to form an equivalent for the p-type conductive layers 6 and 7 without any heat treatment, is disclosed in Jpn. Pat. Appl. KOKAI Publication No. 8-125222. According to this publication, the p-type conductive layers are obtained by replacing an atmospheric gas with inactive gas other than gaseous hydrogen and gaseous ammonia in the process of decreasing the temperature after the growth. This method requires no heat treatment process; however, as described above, a high-resistance layer is formed in the growing layer due to the residual hydrogen or the like, so that it is not thought that good ohmic contact can be obtained or the device characteristics is improved. Furthermore, the publication teaches an Mg-doped layer grown in the nitrogen atmosphere, and the Mg-doped layer is an insulator whose resistance is 10.sup.8 .OMEGA.cm or higher.
The above-described nitride system semiconductor device and its manufacturing method necessitate a specific operation such as heat treatment and electron irradiation after the growth in order to activate magnesium. Therefore, they have problems of decreasing in yield and deteriorating in quality of crystal due to the increase in manufacturing step as well as problems of consuming much time, decreasing in productivity, and increasing in costs.
Furthermore, there is a problem in which since a large amount of hydrogen, carbon and oxygen is present on the surface of the growing layer due to the heat treatment after the growth, the contact resistance of the p-type GaN contact layer 7 rises and the operation voltage of the device increases. There is also a problem in which the device is decreased in reliability since the lifetime of the LED does not reach a practical level due to deterioration of the vicinity of the p-side electrode 9.
The LD has a problem wherein if the resistance of contact with the p-side electrode 9 is high, a laser beam is difficult to emit and, like the LED, it has a problem of remarkably shortening the lifetime due to deterioration of the p-side electrode 9.